Help Me Understand: Why Do Soldiers Out of Step on Bridges?

[emma123]

hi my math teacher ask us this the other day..."in the military, as marching soldiers approach a bridge, the command route step is given. the soldiers then walk out of step with each other as they cross the bridge. explain." i don't understand what she means at all. is it even a physics concept that i don't recognize. please help me .

 

[Lyuokdea]

think of what happens when you stomp on a bridge in terms of forces. Think about the rhythm of marching, and how that could possibly affect a bridge if the timing is right.

The idea is that potentially, the soldiers could accidently march at a speed that is the harmonic frequency of the bridge. If this would happen, the step of many soldiers in time would cause the bridge to vibrate at it's harmonic frequency, and because the frequency is harmonic, the bridge would start to oscillate out of control and possibly break.

Mythbusters did a great episode on this, trying to replicate the effect with piston driven footmen soldiers marching on a harmonic tone of their contstructed bridge, they failed to get the bridge to break up in any harmonic way, although their pistons were a little too powerful and did a great job of breaking the bridge on their own.

However, harmonic frequencies in bridge design cannot be ignored, as they are what led to the Collapse of the Tacoma Narros Bridge in 1940. http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge A dramatic video of the bridge collapse is available here: http://www.archive.org/details/Pa2096Tacoma

~Lyuokdea

 

[kyle8921]

I am not an expert on military tactics or marching formations. However, from a physics perspective, there could be a few reasons why soldiers may walk out of step on bridges.

Firstly, bridges are designed to be strong and sturdy, but they can still have some degree of flexibility. When a large number of soldiers are marching across a bridge in unison, their collective weight and movements can cause the bridge to vibrate. This vibration can be amplified if they are marching in step, which could potentially lead to structural damage to the bridge. By walking out of step, the soldiers are reducing the chances of this vibration and protecting the integrity of the bridge.

Another reason could be related to the spacing between soldiers. When marching in step, soldiers typically have a consistent distance between them. However, on a bridge, this distance may need to be adjusted to account for the width of the bridge and any potential hazards. By walking out of step, the soldiers can adjust their spacing and ensure they are safely crossing the bridge without bumping into each other.

It is also possible that the command to walk out of step on bridges is simply a safety precaution. Marching in step requires a significant amount of focus and coordination, and on a narrow bridge, this could increase the risk of soldiers losing their balance and potentially falling off. By walking out of step, soldiers can maintain their balance and avoid any accidents.

Overall, the command to walk out of step on bridges may not be a physics concept, but it is likely a practical and strategic decision made by military leaders to ensure the safety and success of their troops.